Centrifuges are now well known and widely used in biological sample separation. Centrifuges utilize an abundant, readily available and easily understood force, gravity (G-force) in a relatively simple device called a centrifuge to facilitate particle/fluid separation on the bases of mass and density.
Centrifugation in life sciences involves a smooth, uniform application of G-forces resulting in high yield separation expressed as percentage of recovered target compound from the initial mixture at the end of the separation process. In addition, the separation takes place in a natural fluid, providing a gentle, homogenous cushion during centrifugation. Centrifugation helps maintain bioactivity of the target compound by preserving its conformation or three-dimensional shape. Maintaining both molecular and conformational structure of complex biomaterials is a very important strategy since a large number of biologically important compounds exist in less than nanogram quantities in nature.
This application relates to so-called "fixed-angle" rotors. In a fixed-angle rotor, multiple counterbalanced cavities are used to hold the sample mixture. These cavities are oriented at an angle in fixed-angle rotors with respect to the spin axis of the rotor. Bottles or test tubes containing the starting mixture are placed in the rotor cavities, and rotor is secured over a spindle attached to the centrifuge drive system. Centrifugation occurs.
After spinning the rotor at a predetermined speed for a period of time, the bottles with separated particles are taken out of the rotor. Then, both the supernatant and the sediments are removed from the bottles for further processing and experimentation.
Separation of small biomolecules require fast spinning in order to generate the required g-forces. Today's ultra class centrifuges reach over 100,000 rpm speeds and generate gravitational forces in excess of 600,000.times.Gs.
Containment of such high levels of force requires construction of structurally strong centrifuge rotors. Today, for high speed class centrifuges (with up to 30,000 rpm top speed), rotors are typically subjected to forces less than 100,000.times.Gs. Relatively light-weight and inexpensive, aluminum is used to manufacture high speed rotors.
Although more expensive and heavier, titanium is preferred for ultra centrifuge rotors because of its higher strength and higher corrosion resistance.
In both cases, however, constructing centrifuge rotors from metals poses a serious safety risk associated with their failure mode. Operational or fatigue-related structural break up of metal rotors during spinning can inflict heavy damage to the centrifuge, to the lab or even hurt the operator. In addition, some popular large rotors weigh as much as 60 pounds, raising serious back and spinal injury potential during handling.
During centrifugation, forces generated in rotors are centrifugal. Specifically, major forces normal or perpendicular to the spin axis of the rotor are encountered. For this reason, certain types of rotors have in the past been constructed from composite materials.
Composite materials have the property of being so-called "anisotropic." Specifically, such materials are fabricated tape or fabric laminates containing high tensile strength fibers such as carbon or Kevlar.RTM., a registered trademark of the Du Pont Corporation of Wilmington, Del. The fibers in such materials can be uni-directional in a material commonly referred to as a composite "tape"--that is running in one direction only. In this case the material can be said to be "anisotropic" in its strength; the material has high tensile strength in the direction of the fibers but is otherwise weak normal to the fibers.
Additionally, the fibers in such materials can be multi-directional in a material typically referred to as composite "fabric"--that is running in more than one direction. In this case the material can be said to have bi-directional or multi-directional anisotropic properties in its strength; the material has high tensile strength in the direction of the fibers, which by definition runs in more than one direction.
In the case of centrifuge rotors, it is known to build multiple discrete layers of such material into discs or "billets" of such fabric or tape. Each successive billet layer is rotated with respect to the previously placed layers. The layers are typically fastened one to another. And when such fastening occurs, adjacent layers with anisotropic properties in one direction impart their resistance to adjoining layers with their anisotropic properties aligned in different directions. The discs or billets can in turn be assembled to form a finished article. The finished article can be said to be "quasi-isotropic."
The fabric or tape is built up of multiple discrete layers with the fibers of the fabric normal to the spin axis of the rotor. The fibers of different layers are aligned in different directions. When sufficient discs or billets are joined, they are configured with the requisite hub for spinning and sample tube apertures for containing samples to be centrifuged. A quasi-isotropic rotor structure results with strong properties normal to the spin axis.
Unfortunately, such rotors are relatively weak parallel to the spin axis; they can readily de-laminate at the individual layers when forces parallel to the spin axis are generated between the individual laminates of the rotor.
Composite materials have heretofore been generally unsuitable for use with so-called "fixed angle rotors." Specifically, in such fixed angle rotors the sample tubes are inserted from the top of the rotor into apertures that are angularly inclined at a "fixed angle" with respect to the spin axis. Such angular inclination usually tilts from a point of insertion of the sample tube at the top of the rotor parallel to but away from the spin axis of the rotor. Tilting is usually in the range of 5.degree. to 30.degree.. The bottom of the sample tube is further away from the spin axis of the rotor.
During centrifugation, the sample tube when subjected to centrifugal force exerts a force along the sample tube aperture within the rotor. This force has a component into the body of the rotor along the axis of the sample tube aperture in the rotor. In the case of a composite rotor, this force is the main cause of de-lamination.
For this reason, composite rotors have heretofore been restricted to rotors whose sample tubes are disposed parallel to the spin axis of the rotor where there is no such delaminating force generated. See for example, Piramoon et al U.S. Pat. No. 4,738,656 issued Apr. 19, 1988 entitled Composite Material Rotor; Piramoon U.S. Pat. No. 4,781,669 issued Nov. 1, 1988 entitled Composite Material Centrifuge Rotor; Piramoon U.S. Pat. No. 4,790,808 issued Dec. 13, 1988 entitled Composite Material Centrifuge Rotor; Piramoon U.S. Pat. No. 5,057,071 issued Oct. 15, 1991 entitled Hybrid Centrifuge Rotor; U.S. Pat. No. 5,206,988 issued May 4, 1993 entitled Hybrid Ultra-Centrifuge Rotor with Balancing Ring and Method of Manufacture. All of these particular rotors relate to so-called vertical tube rotors; where the tubes containing the sample are aligned parallel to the spin axis of the rotor.
In an International Application published under the Patent Cooperation Treaty on Dec. 23, 1993 entitled Fix-Angle Composite Centrifuge Rotor by Malekmadani et al., a fixed angle rotor of which I am the omitted inventor, is described. In that disclosure a fixed angle rotor with oblique windings on a conical exterior was provided. The purpose of the oblique windings is to provide a composite rotor with resistance to the forces of vertical separation generated by sample tubes in fixed angle rotors. Rotors having this configuration have been sold more than one year prior to the filing of this patent application; consequently the rotor described in this publication is prior art to this patent application.
There are two phases to fabrication of a centrifuge rotor from composite material. The first phase involves laminating a composite billet or disc as the main structural part. Lamination is done with composite tape or fabric which is laid-up and cut. Heretofore, such lamination and cutting in most cases has been done manually.
Reinforcement of rotors, either of composite construction or conventional construction, has included the winding of such rotors with tows of composite material fibers. Such windings contain as many of discrete filaments as required by the final product design.
To take full advantage of strength properties of unidirectional fiber, an appropriate amount of tension must be applied to filament tow during winding. During winding, tangential friction (similar to belt pulley type friction) causes the filament to adhere under tension to the surface about which winding occurs. This tension must be maintained after winding of the filament has occurred. Since tensioning a fiber filament over the round, angled frustum shaped surface of a fixed-angle rotor can lead to slippage of the filament, such windings have limitations.
In the International Application published under the Patent Cooperation Treaty on Dec. 23, 1993 entitled Fix-Angle Composite Centrifuge Rotor by Malekmadani et al., the disclosed choice is to compromise the tendency of such windings to slip on the frustum shaped or net shaped outer surface by utilizing oblique winding. Because of the requirement to apply fiber tension over the angled surface of the rotor to gain the required vertical reinforcement, an oblique winding was used. This required the use of complex four-axis winding machines. Unfortunately, usefulness of oblique winding in rotor fabrication is limited. Oblique winding over the angular, rounded surface of an angled rotor does not permit application of sufficient filament tow tension and reinforcement. Hence, obliquely wound rotors will not be structurally strong enough for high-load, high-G force applications. Vertical slippage of the wound fibers followed by loss of fiber tension will result in separation of the laminates of the rotor under moderate levels of load and forces. For this reason, this technique has limitations.
Other approaches of reinforcement against forces of vertical separation have been suggested. These approaches include braiding/molding and compression foam molding combinations. While these techniques may be suitable for lower speed, lower G-force centrifuge applications, their usefulness has distinct limitations. These limitations include difficulty of application and manufacture as well as failure under high G loads such as those encountered in ultra speed centrifuge rotors.